Ortho Exam 3

Question 1

Define the relationship b/w stress & strain

Answer 1

Internal state of the material
* calculated from force & deflection

Question 2

Stress

Answer 2

• internal distribution of the load
• Force per unit area

Question 3

Strain

Answer 3

• Internal distortion produced by the load
• deflection per unit length

Question 4

Beams:

Answer 4

Arch wires or springs
1. Cantilever: supported at 1 end only
2. Support Beams: supported at both ends

Question 5

If a Force is applied to a beam, how is the response measured?

Answer 5

Deflection (bending or twisting) produced by the force

Question 6

Define the relationship b/w Force & Deflection

Answer 6

External Measurements

Question 7

What are the 3 major properties of Beam/elastic materials?

Answer 7

• Strenggth
• Stiffness
• Range

Each can be defined by reference to a force-deflection or stress-strain diagram

Question 8

Force-Deflection Diagram

Answer 8

• Stiffness: Slope of linear (elastic) portion
• Range: Distance along the x-axis to Yeild Point
• Yield Point: 0.1% of permanent deformation occured
• Springback: if the wire is deflected beyond the yield point; Does NOT return to original shape
• Failure Point: Wire breaks

Question 9

Stress-Strain Diagram

Answer 9

Strength: the maximum load that the material can resist
* Represented by 3 different points: Proportional Limit, Yield Strength, Ultimate tensile Strength

Proportional LImit: most conservative measure
* The highest point where stress & strain still have a linear relationship (Hooke’s Law)
* Difficult to determine, yield strength is better indicator

Yield Strength: The intersection of
* stress-strain curve
* parallel line offset at 0.1% strain

Ultimate Tensile Strength: The max load a wire can sustain
* > yield strength
* Determines the max force the wire can deliver if used as a spring

Modulus of elasticity (E):
* slope of stress-strain diagram
* porportional to stiffness (E) and Springiness (1/E)

Question 10

Modulus Of elasticity (E)

Answer 10

Slope of stress-strain diagram
* proportional to stiffness (E) and Springiness (1/E)

Question 11

Elastic Limits of Materials

Answer 11

where permanent deformation is 1st observed
* b/w yield strength abnd proportional limit

Question 12

Ultimate Tensile Strength

Answer 12

The max load a wire can sustain
* > yield strength
* Determines the max force the wire can deliver if used as a spring

Question 13

Yield Strength

Answer 13

The intersection of
* stress-strain curve
* parallel line offset at 0.1% strain

Question 14

Proportional limit

Answer 14

most conservative measure
* The highest point where stress & strain still have a linear relationship (Hooke’s Law)
* Difficult to determine, yield strength is better indicator

Question 15

Define the relationship b/w strength, stiffness, and range

Answer 15

Strength= Stiffness x Range

Question 16

Strength

Answer 16

The max load the material can resist

Measure in units of stress:
* SI Unit: Pascal (Pa) or MPa (Mega Pascal)
* gm/cm^2
* psi

Question 17

Stiffness

Answer 17

Proportional to the slope of the linear portion (Elastic portion) of force-deflection curve
* The more vertical the slope: the Stiffer the wire
* The more horizontal the slope: the more flexible the wire

Question 18

Range

Answer 18

The distance that we wire will bend elastically before permanent deformation occurs
* measured in mm
* If the wire is deflected beyond this point, does not go back to original shape

Question 19

Springback

Answer 19

Determines Clinical Performance
* Measure along the x-axis
* if the wire is deflected beyond the yield point->Does NOT return to original shape

Question 20

Resilience

Answer 20

Area under the stress-strain curve->out to the proportional limit
* Energy capacity of the wire: combo of strength & springiness

Question 21

Formability

Answer 21

Amount of permanent deformation a wire can withstand before failing
* amount of permanent bending the wire will tolerate before it breaks
* distance along x-axis

Question 22

Properties of an ideal wire for ortho:

Answer 22

• High strength
• Low stiffness
• High range
• High Formability
  Reasonable Cost
  Weldable or Solderable:
• can attach hooks & stops to the wire

Question 23

Precious Metal Alloys

Answer 23

Gold:
* to soft for all dental purposes
* advantage: easy to fabricate cast appliance

Alloys:
* could be used for ortho
* included: Platinum, Pallidium, Gold, Copper

Question 24

Stainless Steel Wire

Answer 24

Replaced Precious metals bc:
* better strength & springiness
* equivalent corrosive resistance

Resistant to rust—due to
* High Chromium Content- 18% Chromium + 8% Nickel= 18-8 Stainless Steel

Steel Properties:
Controlled over a wide range by:
* Cold working-Hard Steel
* annealing-Softer Steel
Fully Annealed Stainless Steel wires (aka “Dead soft”) wire
* Soft
* Highly formidable

Steel archwires offered in a range of partially annealed states
* Yield Strength is progressive enhanced at the cost of formability
1.Super Grades of Steel wires
* most yield strength
* almost brittle- will break if bent to sharply
2.Regular grade of Steel wire
* can bend to any shape w/o breaking

Question 25

Cobalt-Chromium Alloy

Answer 25

Aka Elgiloy
Advantage: can be supplied in a softer state
* thus more formable

Heat treatment:
* harden wires after being shaped;
* Increase Strength
After Heat treatment:
* the softest Elgiloy=regular stainless steel wire
* Harder initial grades=Super Stainless Steel

Rarely used today